Organelle

In cell biology, an organelle is a discrete structure of a cell having specialized functions. There are many types of organelles, particularly in the eukaryotic cells of higher organisms. An organelle is to the cell what an organ is to the body (hence the name organelle, the suffix -elle being a diminutive). Organelles were historically identified through the use of microscopy, and were also identified through the use of cell fractionation.

A few large organelles probably originated from endosymbiont bacteria:

mitochondria (in almost all eukaryotes)

plastids (in plants and algae)

chloroplasts, mature forms of etioplasts

chromoplasts

leucoplasts

amyloplasts

statoliths

elaioplasts

proteinoplasts

rhodoplasts

Other organelles have had endosymbiotic origins suggested for them (notably flagella; see Evolution of flagella), but these theories are not widely accepted.

Eukaryotic organelles

Eukaryotes
are the most structurally complex known cell type, and by definition
are in part organized by smaller interior compartments, that are
themselves enclosed by lipid membranes that resemble the outermost cell membrane. The larger organelles, such as the nucleus and vacuoles,
are easily visible with moderate magnification (although sometimes a
clear view requires the application of chemicals that selectively stain
parts of the cells); they were among the first biological discoveries
made after the invention of the microscope.

Not all eukaryotic cells
have all of the organelles listed below, and occasionally, exceptional
species of cells are missing organelles which might otherwise be
considered universal to eukaryotic cells (such as mitochondria).
There are also occasional exceptions to the number of membranes
surrounding organelles, listed in the tables below (e.g. some which are
listed as double-membraned are sometimes found with single or triple
membranes).

Major eukaryotic organelles

Organelle

Main function

Structure

Organisms

Notes

chloroplast (plastid)

photosynthesis

double-membrane compartment

plants, protists

has some genes

endoplasmic reticulum

modification and folding of new proteins and lipids

single-membrane compartment

all eukaryotes

Golgi apparatus

sorting and modification of proteins

single-membrane compartment

most eukaryotes

mitochondrion

energy production

double-membrane compartment

most eukaryotes

has some genes

vacuole

storage & homeostasis

single-membrane compartment

eukaryotes

nucleus

DNA maintenance & transcription to RNA

double-membrane compartment

all eukaryotes

has bulk of genome

Organelles which have double-membranes and their own DNA are believed by many biologists of having originally come from incompletely consumed or invading prokaryotic cells, which were adopted as a part of the invaded cell through endosymbiosis.

Originally, the word organelle referred to large lipid bags within cells; later, as other cell parts were discovered, the meaning was extended to also include smaller parts of cells.

Other eukaryotic organelles and cell components

Organelle

Main function

Structure

Organisms

acrosome

helps spermatoza fuse with ovum

single-membrane compartment

many animals

centriole

anchor for cytoskeleton

Microtubule protein

animals

cilium

movement in or of external medium

Microtubule protein

animals, protists, few plants

glyoxysome

conversion of fat into sugars

single-membrane compartment

plants

hydrogenosome

energy & hydrogen production

double-membrane compartment

a few unicellular eukaryotes

lysosome

breakdown of large molecules

single-membrane compartment

most eukaryotes

melanosome

pigment storage

single-membrane compartment

animals

mitosome

not characterized

double-membrane compartment

a few unicellular eukaryotes

myofibril

muscular contraction

bundled filaments

animals

nucleolus

ribosome production

protein-DNA-RNA

most eukaryotes

parenthesome

not characterized

not characterized

fungi

peroxisome

oxidation of protein

single-membrane compartment

all eukaryotes

ribosome

translation of RNA into proteins

RNA-protein

eukaryotes & prokaryotes

vesicle

miscellaneous

single-membrane compartment

all eukaryotes

Other related structures:

cytosol

endomembrane system

nucleosome

microtubule

cell membrane

Prokaryotic organelles

Prokaryotes are not as structurally complex as eukaryotes, and do not have any compartments enclosed by lipid membranes.
In the past they were often viewed as having little internal
organization, but slowly details are emerging about prokaryotic
internal structures. One contributing discovery was that at least some
prokaryotes have microcompartments, which are compartments enclosed by proteins.